Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Mittwoch
TT 19 Supraleitung: Heterostrukturen, Andreev-Streuung, Proximity-Effekt, Koexistenz
Zeit: Mittwoch 16:45–18:45 Raum: H20
Hauptvortrag TT 19.1 Mi 16:45 H20
Triplet correlations in superconductor-ferromagnet hybrid
structures — •Matthias Eschrig, Juha Kopu, Juan-Carlos
Cuevas, A. Konstandin, and Gerd Schön — Institut für
theoretische Festkörperphysik, Universität Karlsruhe
Based on the technique of quasiclassical Green’s functions, we construct
a theoretical framework for describing heterostructures consisting
of superconductors and ferromagnets. We show, that singlet-triplet
mixing near such interfaces is important and affects strongly the proximity
of superconductivity into strong ferromagnets. We suggest a new
mechanism for the penetration of superconducting correlations into halfmetallic
materials via an indirect proximity effect, which involves equal
spin pairing correlations. We discuss the influence of magnetic domain
walls on the superconducting proximity effect.
TT 19.2 Mi 17:15 H20
Ferromagnetic/Superconducting Bilayer Structure: A Model
For Spin Diffusion Length Estimation — •Soltan Soltan,
Joachim Albrecht, and Hanss-U. Habermeier — Max Planck
Institut für Festkörperforschung, Heisenbergstr.1, D 70569 Stuttgart,
Germany
Epitaxial bilayer structures of La2/3Ca1/3MnO3 (LCMO) and
YBa2Cu3O7−δ (YBCO) are grown on single crystalline SrTiO3 substrates
by pulsed laser deposition. The microstructure is analyzed by XRD and
exhibits c-axis oriented growth. The properties of these samples are investigated
by current transport and magnetization measurements. We
find that the transition temperature of the superconductor strongly depends
on the thickness of the YBCO film which can be explained by the
tunnelling of spin polarized quasiparticles from the magnetic film into
the superconductor. A theoretical model has been developed that fits the
experimental data quite well and allows us to determine the spin diffusion
length to be in the order of 10 nm. Furthermore we observe a shift
of the Kosterlitz-Thouless transition temperature close to Tc in case of
the injection of an additional transport into the bilayers.
TT 19.3 Mi 17:30 H20
Measurement of nonlocal conductance in superconductorferromagnet
hybrid structures — •Detlef Beckmann 1 , Heiko
B. Weber 1 , and Hilbert v. Löhneysen 2,3 — 1 Forschungszentrum
Karlsruhe, Institut für Nanotechnologie — 2 Forschungszentrum
Karlsruhe, Institut für Festkörperphysik — 3 Physikalisches Institut,
Universität Karlsruhe
We have measured the conductance of nonlocal aluminum-iron spinvalve
structures fabricated by e-beam lithography and shadow evaporation.
The sample geometry consists of an aluminum wire with two or more
ferromagnetic wires forming diffusive point contacts to the aluminum at
varying distances from each other. In the normal state of aluminum,
we observe a spin-valve signal which allows us to control the relative
orientation of the magnetizations of the ferromagnetic contacts. In the
superconducting state, at low temperatures and excitation voltages well
below the gap, we observe a spin-dependent non-local conductance which
decays on a smaller length scale than the normal-state spin-valve signal.
The sign, magnitude and decay length of this signal is consistent with
predictions made for crossed Andreev reflections [1] (CARE) in a diffusive
superconductor.
[1] G. Deutscher and D. Feinberg, Appl. Phys. Lett. 76 (2000) 487, D.
Feinberg, cond-mat/0307099.
TT 19.4 Mi 17:45 H20
Spontaneous Current in a Superconducting Loop with Ferromagnetic
Josephson Junction — •Andreas Bauer 1 , Johannes
Bentner 1 , Marco Aprili 2,3 , Maria-Luisa Della Rocca 2,3 ,
Matthias Reinwald 1 , Werner Wegscheider 1 , and Christoph
Strunk 1 — 1 Institut für Experimentelle und Angewandt Physik,
Universität Regensburg, Germany — 2 CSNSM-CNRS, Université
Paris-Sud, Orsay, France — 3 LPQ-ESPCI, 75005 Paris, France
Recently it has been shown that Cooper pairs can be transferred coherently
across a very thin ferromagnetic interlayer sandwiched between
two superconducting reservoirs. Thereby an exchange field acts upon the
spins of the paired electrons, resulting in a spatially oscillating pair amplitude.
For proper values of the ferromagnetic layer thickness and exchange
field, this oscillation allows the construction of Josephson-junctions with
a built-in phase difference of π. Niobium loops interrupted by a PdNi
π-junction are fabricated by means of shadow evaporation using a high
temperature stable shadow mask made of Polyethersulfone and Germanium.
A single loop is placed on top of the active area of a micro-Hall
sensor made of a modulation doped GaAs/AlGaAs heterostructure. By
varying the applied magnetic field, the phase difference across the weak
link is tuned. When comparing the magnetic response of loops with and
without π-junction, the π-loop is found to be asymmetric when reversing
the applied magnetic field. Upon cooling down the loop below the critical
temperature in zero field, a spontaneous current is detected which
provides half-integer flux quantization in the π-loop. Both effects can be
understood in terms of the intrinsic phase bias of the π-junction.
TT 19.5 Mi 18:00 H20
Experimental detection of the current phase relation in SNS
Josephson contacts — •Johannes Bentner, Andreas Bauer,
Matthias Reinwald, Werner Wegscheider, and Christoph
Strunk — Institut für experimentelle und angewandte Physik,
Universität Regensburg, Universitätsstrasse 31, 93053 Regensburg
Short superconductor/normalconductor weak links (SNS) are expected
to show a non-sinusodial current phase relation. This is due to a relativ
high transmission probability, and hence the Cooper pairs are not transfered
uncorrelated, but in Clusters of n correlated pairs. We present direct
measurements of the current phase relation of Al/Au/Al and Al/Ag/Al
SNS weak links prepared by shadow evaporation. Micropatterned GaAs-
AlGaAs Hall-sensors are used to measure the flux which is generated by
the circulating supercurrent in Al loops containing a single SNS junction.
The supercurrent is generated by the phase difference across the
junction which is controlled by applying an external magnetic flux to
the loop. In highly transparent Al/Ag junctions we find a non-sinusoidal
current phase relation. In contrast to that, in Al/Au junctions with lower
transparency the standard sinusoidal behavior is found. The critical current
extracted from the measured current phase relations agrees well with
transport measurements of simliar control samples.
TT 19.6 Mi 18:15 H20
Parity Effect in SN-Proximity-Systems — •Harald Kloos and
Andrei Zaikin — Forschungszentrum Karlsruhe, Institut für Nanotechnologie,
76021 Karlsruhe, Germany
A normal metal layer in good electric contact to a superconductor exhibits
properties sumarized under the term proximity effect, i.e. Andreev
reflection and the formation of a gap in the density of states which is
much smaller than the superconducting order parameter ∆. At low temperatures
thermodynamic properties of small isolated superconducting
grains depend on the parity of the number of electrons they carry, because
pairs of electrons can form Cooper pairs and enter the condensate,
whereas the lowest available state for an unpaired electron is given by
∆. We investigate the parity effect in an SN layer structure within the
quasiclassical Green’s functions formalism for arbitrary concentrations of
nonmagnetic impurities. The temperature T ∗ is estimated, below which
parity effects dominate the physical properties of the sample due to the
proximity induced features of the N-layer. It is shown that the free energy
difference between ensembles with even and odd numbers of particles at
zero temperature coincides with the proximity gap of the normal metal
layer. At finite temperatures the entropy factor is related to the density
of states of the sample.
TT 19.7 Mi 18:30 H20
Parity-Affected Supercurrents in Hybrid Nanorings —
•Sergei Sharov 1 and Andrei Zaikin 2 — 1 Institut für Theoretische
Festkörperphysik, Universität Karlsruhe, 76128 Karlsruhe, Germany
— 2 Forschungszentrum Karlsruhe, Institut für Nanotechnologie, 76021,
Karlsruhe, Germany
It has been established both experimentally and theoretically that
thermodynamic properties of small isolated superconducting islands and
grains may strongly depend on the electron parity number. In the present
work we investigate the influence of the parity effect on supercurrents in
isolated superconducting nanorings interrupted by a quantum point contact(QPC)
and threaded by the Aharonov-Bohm magnetic flux.
In order to evaluate superurrents in rings with even and odd total